1. Field of the Invention
This invention relates to luminescent screens of at least two crystalline layers; more particularly, screens in which a mismatch in lattice constant of a sublayer and overlayer provide enchanced luminescent output.
2. Description of the Prior Art
The use of rare-earth phosphor materials in cathodoluminescent screens for cathode ray tube applications is well known. One disadvantage of the conventional, powder phosphor layer in cathode ray tubes is its degradation due to heating effects from the energy absorbed from the electron beam. A second disadvantage is that the screen resolution is limited by the size of the phosphor particles and the non-uniformity of the deposited phosphor layer.
Single crystal phosphor layers on suitable substrates solve both these problems. The intimate contact between an epitaxial phosphor layer and its substrate facilitates heat transfer out of the phosphor layer. Since the phosphor is a single crystal, there is no particle-size-limiting resolution. (Such epitaxial single crystal crystal phosphor layers have been reviewed in J. M. Robertson et al, Thin Sol. Films 114, 79221) (1984). (See also U.K. patent application GB2000173A and European patent application No. 79200011.9.) A disadvantage of single crystal phosphor layers is that much of the light that is generated in the cathodoluminescent phosphor is "piped" to the edges of the phosphor layer/substrate composite. This waveguide action reduces the useful light output at angles close to normal viewing incidence.
One method for increasing the intensity of light at normal incidence, disclosed in U.S. Pat. No. 4,298,820, issued on Nov. 3, 1981, to Bongers et al, is to etch grooves into the phosphor layer to allow the escape of light. A disadvantage of this technique is that the resolution of the phosphor layer is limited by the dimensions of the grooves. Another disadvantage is the extra processing required to form the grooves; some cathodoluminescent materials, such as yttrium aluminum garnet (YAG), are highly resistant to chemical and mechanical milling.
Certain epitaxial growth conditions on garnet result in a type of defect known as faceting. The conditions for such facet growth in a type of magneto-optical garnet were disclosed by D. M. Gualtieri at the 30th annual Conference on Magnetism and Magnetic Materials, San Diego, November 1984 (D. M. Gualtieri and P. F. Tumelty, to be published in J. Appl. Phys.). J. M. Robertson et al. (op. cit., p.227) reported such a facet defect in garnet phosphor layers and observed that facets scatter cathodoluminescence to forward directions. They also disclosed that such a defect reduced the resolution, because of multiple internal reflections, and concluded that facets should be avoided in the preparation of such layers.